88 REPORTS ON THE STATE OF SCIENCE.—1918. 
It will be noted that this theory draws no distinction between 
matters in true and pseudo solution so far as the mechanism of 
the purification changes is concerned. 
Experience with the operation of the Hampton sewage works, 
where relatively large quantities of sludge accumulated in the 
filtration area, led Travis to study the question of sewage colloids 
in relation to sludge production. As the result of a series of 
experiments carried out in conjunction with Johnston, he 
advanced in 1905 what is known as the Hampton Doctrine. 
According to this theory the purification process is primarily 
and essentially a de-solution effect brought about purely by 
physical causes and that any bacterial or biological action is 
definitely ancillary. 
Unlike Dunbar’s theory, a distinction is drawn with regard to 
the mechanism of the purification process as between organic 
matter in colloidal solution and matter in true solution, 
It is held that whereas ‘ organic matters and products in actual 
solution, e.g., ammonia, sulphuretted hydrogen and volatile 
products, are absorbed by the accumulated deposits in the filter,’ 
the solids in colloid solution are coagulated or precipitated ‘ in 
virtue of a physical action ’—effect of extended surface contact, 
&e. 
These coagulated colloids are either retained in the filter or 
discharged with the effluent from the filter, and any biolytic 
action on them is a very slow and prolonged process, and conse- 
quently in addition to the actual suspended solids in the sewage 
or tank-effluent treated, the amount of such coagulated colloids 
which have been termed ‘ ultra sludge,’ must receive attention 
in considering the amount of the total sludge to be dealt with. 
In discussing these theories, it is proposed to confine attention 
as far as possible to the purification effect on matters in colloidal 
solution. 
Briefly, it may be said that— 
(i) In the earlier theory, especially as propounded by Stod- 
dart, that such matters are held to be oxidised directly 
by bacterial agency during the passage of the liquid 
through the filter. 
(ii) That Dunbar’s theory provides for their preliminary 
absorption by the filtering media and the gelatinous 
coating thereon, with subsequent more or less rapid 
oxidation by agency of condensed oxygen, bacteria 
and higher forms of life, 
(111) That according to the Hampton Doctrine, such absorp- 
tive processes are confined to the matters in true solu- 
tion, and that the matter in pseudo solution is 
coagulated by virtue of a physical action. That the 
subsequent biolytic oxidation of the matter thus 
deposited is a very prolonged process. 
In connection with the question of absorption, it may be 
mentioned that Clark, and later Clifford, have demonstrated that 
the mean time of passage of sewage through a filter as operated 
in practice, while it varies considerably, dependent on several 
